It would be advantageous to improve infrared (IR) and ultraviolet (UV) absorption of soda-lime-silica glass products while maintaining a high visible transmission. For example, automotive vehicles require glass having high visible transmittance to assure optimum visibility for the operator. Infrared and ultraviolet light do not improve visibility, but generate heat within the passenger compartment and, particularly during summertime driving, increase the load on the air conditioning system to maintain comfort levels.
Iron oxide is commonly added to glass to produce a green color. In glass, iron oxide is found in two chemical forms. The oxidized compound is ferric oxide, Fe2O3, and is yellow. The reduced compound is ferrous oxide, FeO, and is blue. Advantageously, ferric oxide absorbs a portion of ultraviolet light passing through the glass product; and ferrous oxide absorbs a portion of infrared light passing through the glass product. Under typical furnace melting conditions, when the total iron oxide in the glass product is within the range of about 0.3 to 0.8 weight percent, the iron oxide equilibrium is such that the redox ratio of FeO to total iron oxide is about 0.23 to 0.26, which imparts a green color to the glass. As used herein, total iron oxide refers to weight of an equivalent amount of iron as ferrous oxide, Fe2O3. Also, as used herein, compositional percentages are based upon weight, except as otherwise noted.
During melting, it is common practice to add a sulfate compound, typically sodium sulfate, and a carbonaceous material, typically anthracite coal, for refining purposes. In the presence of carbon, the sulfate compound dissociates to form sulfur oxide that facilitates the removal of bubbles from the molten glass, which would otherwise produce defects in the product.
It is also desirable to produce glass having a dark blue coloration for aesthetic purposes. It is known that increasing the proportion of ferrous oxide relative to ferric oxide shifts the glass color from green to blue. This is readily accomplished by increasing the addition of carbonaceous material to the glass melt, whereupon the additional carbon reacts with ferric oxide to form additional ferrous oxide. However, decreasing the ferrous oxide reduces infrared absorption by the glass. Moreover, attempts to compensate by increasing the total iron concentration to maintain a high infrared absorption reduces visible transmittance of the glass and is not desired. This is attributed, in part, to a reaction between iron and sulfur derived from the sulfate refining agent to produce iron sulfide, which imparts an amber coloration that dramatically decreases visible transmittance and also shifts the color of the glass so that the desired aesthetically blue coloration is not achieved. It is also known to produce blue glass by additions of cobalt oxide. However, when added to glass containing iron sulfide, the amber coloration shifts the dominant wavelength away from the desired blue range and reduces visible transmittance.
Therefore, a need exists for a glass having enhanced blue coloration as indicated by a high excitation purity that is not diminished by iron sulfide amber or other non-blue coloration, and which further exhibits a high visible transmittance and high infrared absorption.